The present invention relates to a control system for four-wheel drive vehicles, and more particularly to a system for controlling the torque capacity of a transfer clutch for four-wheel drive vehicles.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive power transmission train to a four-wheel drive Cpower transmission train by engaging the transfer clutch. Meanwhile, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" will occur at a sharp corner. Japanese Patent Application Laid Open No. 57-84227 discloses a system which is intended to eliminate the braking phenomenon. The disclosed system is provided with a fluid operated friction clutch as a transfer clutch, and a four-wheel drive power transmission system is converted to a two-wheel drive power transmission system by disengaging the clutch when a vehicle turns a sharp corner. However, when oil is discharged from the clutch, pressure of oil in the clutch decreases quickly. Accordingly, the torque transmitted by the clutch changes quickly and largely, which causes a great shock in the vehicle at the change of the power transmission train.